Power systems generally include power converting systems, for example inverter or rectifiers. These converters may be employed in safety critical applications such as starter generators, e-oil, e-fuel or electrical actuation systems in aero applications. In these applications, safety and reliability are of the utmost importance. They can also be used in the context of land and sea electrical/hybrid electric propulsion systems, as well as for industrial drives, solar and wind power conversion etc.
It is estimated that around 38% of the faults in the variable speed drives used in these sectors are due to failure of the power devices (F. W. Fuchs, “Some diagnosis methods for voltage source inverters in variable speed drives with induction machines a survey,” in Proc. 29th Annu. Conf. IEEE Ind. Electron. Soc., 2003, pp. 1378-1385). The reliability of the power devices has improved in the past decade, in terms of progressive designs and optimized use of materials. However for mission-critical applications such as aerospace the reliability of the power devices may not be satisfactory, particularly as these sectors are prone to unpredictable failure events (K. Keller, K. Swearingen, J. Sheehan, M. Bailey, J. Dunsdon, K. W. Przytula and B. Jordan, “Aircraft Electrical Power systems prognosis and Health Management”, IEEE Aerospace Conference Proceedings, Big Sky, MT, March 2006). In particular, there is an increased risk of premature failure when the devices are operated in harsh conditions which accelerate the aging process of the devices. Moreover, in safety critical applications a premature failure could lead to complete shutdown and require expensive and unplanned maintenance. Therefore health monitoring of these devices can is desirable.
Previously, the health of power devices has been estimated based on the threshold voltage limit of the on-stage voltage of the IGBT (Ralf Schmidt, Felix Zeyss, and Uwe Scheuermann. Impact of absolute junction temperature on power cycling lifetime. 15th European Conference on Power Electronics and Applications (EPE), pages 1-10, 2013). Accelerated aging tests were performed to create thermal stress tests of the devices to induce latch-up failures, and in particular through increasing the maximum junction temperature of a given device by reducing its heat transfer capability (for example by removing the heat sink). The device was then subjected to high temperature cycling until the latch-up failure occurred. In parallel to this, the junction temperature of the device was estimated based on the power loss. The estimated junction temperature and temperature cycles were noted, and from these (and the measured threshold voltage limit) the remaining useful life (i.e. health) was estimated. Unfortunately, the variation of the threshold voltage during degradation of the device can be lost amongst signal noise. This method also requires measurements at the high voltage side, and therefore voltage clamping is required to measure the voltage accurately, which is undesirable.
Another approach for monitoring the health of power devices is based on the switching behaviour. It utilizes the changes in ringing behaviour during the commutation period of a power module. This requires measuring the voltage or current in the high voltage side of the inverter, and uses a bandpass filter to extract the high frequency ringing component. This approach relies on the fact that increase in the damping co-efficient and attenuation in high frequency ringing of voltage and current waveform occurs after ageing of the power modules.